This invention relates to the equalisation of optical signals in an optical communications system and in particular but not exclusively to the equalisation of wavelength division multiplexed optical signals.
The control of optical power levels in optical communications systems is critical in obtaining optimum performance since the power level should be sufficient to establish a signal to noise ratio which will provide an acceptable bit error rate but without the power level exceeding a level at which limiting factors such as the onset of non-linear effects result in degradation of the signal. In wavelength division multiplexed (WDM) transmission, it is desirable to maintain each of the power levels of the individual wavelength components at substantially the same level.
The inventor has disclosed in U.S. Pat. No. 5,513,029 a method of monitoring component power levels in WDM transmission using orthogonal low frequency dither signals and controlling component signal power to maintain optimum performance.
It is also known from GB2314714A that an imbalance of component signal powers in a WDM transmission is likely to occur at an optical amplifier stage, as used to boost signal power at stages in a long distance transmission, utilising optical amplifiers such as erbium doped fibre amplifiers. Such amplifiers have a non-uniform gain characteristic as a function of wavelength which is variable in dependence on the amplifier gain, this change in gain characteristic consequent on change of gain being commonly referred to as dynamic gain tilt.
There is therefore a need to provide optical filtering which is adaptive and which can be used in conjunction with optical amplifiers, or otherwise, in order to maintain a preferred spectral profile of an optical signal.
It is known from Huang et al, IEEE Photonics Technical Letters, September 1996 pp 1243-1245, to provide an acousto-optic tunable filter for dynamic equalization of channel powers. A disadvantage of such a method is that the filters suffer from polarisation sensitivity and severe channel cross talk.
It is also known from Gobel et al, IEEE Photonics Technology Letters, March 1996, pp 446 to 448, to provide a WDM power level compensator in which demultiplexed channels are subject to power control in respective erbium doped waveguides. A disadvantage of this arrangement is that significant distortion of the modulated optical signal occurs.
It is also known from Madsen et al, IEEE Journal of Lightwave Technology, March 1996, pp 437 to 447, to provide fixed (non-adaptive) filters using a sequence of concatenated Mach-Zehnder interferometers in a planar waveguide structure. Such structures require lengths which are difficult to fit onto a single planar waveguide structure and which have an inherent high insertion loss.
Parallel structures on planar waveguides are known from Dragone, IEEE Photonics Technology Letters, September 1991, pp 812 to 815, which provide non-adaptive filtering with output at a single wavelength.
It is also disclosed by S. Day in co-pending application U.S. Pat. No. 08/997,752, now U.S. Pat. No. 5,956,437, to provide a variable optical attenuator by means of localised heating of a waveguide.
Yamada et al, Electronics Letters 1995, 31, pp 360 to 361, discloses a multiplexer using planar waveguide technology and in which a waveguide array is provided with heating strips for each waveguide in order to compensate for phase errors occurring during fabrication. After such compensation, light components passing through the arrayed waveguides are delayed by respective amounts which differ by a constant phase difference between adjacent waveguides so that recombination in a star coupler at the output of the waveguides is dispersive in wavelength to provide separation of the WDM channels, this arrangement thereby being termed an arrayed waveguide grating.
There remains a need to provide an improved optical equalizer, particularly for use in the context of correcting gain tilt in optical amplification stages of a communications system.
It is an object of the present invention to provide optical equalisation to at least partially compensate for the effects of gain tilt in optical amplifiers.
It is a further object of the present invention to provide an adaptive optical equalizer using planar waveguide technology.
It is a further object of the present invention to provide a method of applying a desired frequency characteristic of equalization by actuating a minimal number of elements of an adaptive optical equalizer.
It is a further object of the present invention to allow the operation of an optical equalizer to be periodically refined to adapt to changing conditions and system tolerances.
According to the present invention there is disclosed a method of applying equalization to an optical signal for use in an optical communications system, comprising steps of:
splitting the optical signal into components having the same frequency characteristic as the optical signal;
transmitting the components via respective waveguides of a waveguide array defining respective optical path lengths;
variably setting at least one of the relative amplitudes of and the phases of the components transmitted via the waveguides; and
combining the components transmitted by said waveguides to form an output optical signal whereby interference between said combining components applies equalization to the frequency characteristic of the output optical signal.
According to a further aspect of the present invention there is disclosed a method of controlling an adaptive filter having a set of elements configured such that their combined effect determines a frequency characteristic of equalization applied by the filter; the method comprising the steps of;
actuating the elements according to values of a corresponding set of parameters;
calculating said parameters from complex values of control coefficients by a process which includes a discrete Fourier transform; and including the step of inputting values of amplitude for the control coefficients and calculating respective phase values of said coefficients by a Hilbert transform.
According to a further aspect of the present invention there is disclosed a method of operating an optical filter to apply a desired equalization characteristic to a wavelength division multiplexed optical signal, the method comprising the steps of;
actuating a set of elements of the filter such that their combined effect determines an actual frequency characteristic applied to the optical signal, said elements being actuated according to values of a corresponding set of parameters;
calculating said parameters from control values determined in accordance with said desired equalization characteristic;
measuring the output of the filter and obtaining a measurement of the actual frequency characteristic;
comparing the measurement with the desired frequency characteristic to obtain difference values;
calculating new values of said control values based on said difference values and actuating said elements according to new parameters calculated from said new control values;
and periodically repeating said steps of measuring, actuating and calculating new values to apply an actual equalization characteristic substantially equal to said desired equalization characteristic.
The present invention allows an optical equalizer to be realized using planar waveguide technology by utilising a waveguide array and relatively simple modulators as control elements for modifying the transmission through each waveguide. The calculation of parameters used to determine actuation of the elements of the equalizer can be optimized by use of a Hilbert transform to reduce the number of non-zero parameters requiring corresponding elements to be actuated in accordance with values of the parameters.
The invention further provides the ability to periodically repeat calculation of parameters and actuation of the elements based on feedback of measured output from the equalizer to achieve improved correspondence with a desired equalization by iteration or to remove the effects of system errors.
Preferred embodiments of the present invention will now be described by way of example only.